Nutrient-induced perturbations to δ13C and δ15N in symbiotic dinoflagellates and their coral hosts

Inorganic nutrients play a critical role in determining benthic community structure in tropical seas. This study examined the impact of adding inorganic nutrients (ammonium and phosphate) on the isotopic composition of 2 reef-building corals, Pocillopora damicornis and Heliofungia actiniformis, on the southern Great Barrier Reef. The addition of elevated nutrients to patch reefs that pond at low tide did not perturb the C:N ratio of either species or their symbiotic dinoflagellates. The C:N ratios were significantly higher in material extracted from the skeleton (14.8 1.50 and 10.8 1.42) than either host (7.6 0.87 and 6.0 0.71) or symbiotic dinoflagellates (5.7 0.48 and 6.9 0.66) (P. damicornis and H. actiniformis respectively; 95 confidence intervals). The ratio of acquired N to background N suggests that the added dissolved inorganic nitrogen (DIN) accounted for 50 to 100% of total nitrogen within the tissues of P. damicornis and H. actiniformis at the end of the experiment. The addition of the isotopically depleted nutrients ( 15 N = 0 ) to patch reefs significantly decreased 15 N from control values of between 3 and 4 to values to below 1 in the case of all compartments, while 13 C values were relatively unresponsive to nutrient treatments. These findings suggest that coral 15 N has the potential to provide a historical record of the 15 N of dissolved nitrogen surrounding reef-building corals and their symbiotic dinoflagellates.